Direct air capture can mitigate the increasing CO2 emissions associated with the carbon polluting sources. Efficient and cost-effective removal of trace CO2 is important in various key industrial applications pertaining to energy, environment and health. From an industry prospective, the removal of trace CO2 from air is a growing area of research and development due to its substantial importance for pre-purification of air and particularly when atmospheric air is used during the separation of nitrogen and oxygen.
The amount of CO2 in the atmosphere continues to rise rather rapidly due to unparalleled cumulative CO2 emissions, provoking the undesirable greenhouse gas effect. Certainly, it is becoming critical to develop economical and practical pathways to reduce CO2 emissions. Appropriately prospective routes to address this enduring challenge have been considered: (i) CO2 emission reduction from post-combustion stationary and mobile sources where CO2 concentration is in the range of 10-15% and (ii) CO2 removal from air, called direct air capture (DAC), which is another alternative option to reduce greenhouse gases emissions in a uniform way globally. Although DAC is relatively more challenging than post-combustion capture, it is recognized that it might be practical, provided that suitable adsorbent combining optimum uptake, kinetics, energetics and CO2 selectivity is available at trace CO2 concentrations.
In an example, prior to air separation using cryogenic distillation or pressure swing adsorption (PSA), air must be CO2 free to avoid (i) blockage of heat-exchange equipment as a result of frozen CO2 during the liquefaction process and (ii) adsorbents (e.g., zeolites) contamination used for oxygen production by pressure swing adsorption (PSA).
Equally important, alkaline fuel cells (AFCs) require a CO2 free feedstock of oxygen and hydrogen gases as it is widely recognized that trace amounts of CO2 (i.e. 300 ppm) degrade the electrolyte in AFCs. Furthermore, efficient removal of CO2 at low concentrations is also vital for the proper operation of breathing systems in confined spaces such as submarines and aerospace shuttles.
Efficient CO2 removal and resupply of fresh air is also critical in mining and rescue missions, diving, and most importantly in medical applications such as anaesthesia machines. The use of anaesthesia machine is still a growing clinical trend worldwide, driven by the need to reduce cost and improve patient care via the use of efficient CO2 sorbents. A CO2 removal feature in anaesthesia machine is particularly important in semi-closed or closed rebreathing systems, as the rebreathing fraction is at least 50% of the exhaled gas volume, directed back to the patient after proper CO2 removal in the next exhalation. Currently, common sorbents for this application are non-recyclable, and generate large amounts of unwanted medical waste.
There is a pressing need to develop novel porous materials that can adequately address the growing interest to low CO2 concentration removal applications. Only a few materials were reported to adsorb efficiently traces of CO2, particularly with regards to DAC using a variety of amine supported materials (e.g. porous silica). However, these materials contain primary amines which require high energy for regeneration, such as about 80-120 kJ/mol, in part due to the materials' chemical adsorption mechanisms. Additionally, amine grafting is conducted in a step separate from the platform material synthesis, thus adding additional cost and time to manufacturing.
Modular and tunable porous materials, namely metal-organic frameworks (MOFs), can be used to tackle this ongoing challenge. Recently, MOFs were intensively investigated for intermediate and high CO2 concentration removal applications such as post-combustion, pre-combustion capture, natural gas and biogas upgrading. Nevertheless, the potential of MOFs to remove traces and low CO2 concentration from gas streams was rarely considered. The main reason for this lack of studies is that most MOFs reported so far, with or without unsaturated metal sites (UMC) or/and functionalized ligands, exhibit relatively low CO2 selectivity and uptake particularly at relatively low CO2 partial pressure. To overcome this downfall, various research groups have adopted the amine grafting chemistry and the acquired knowledge from amine-supported silica, as a prospective pathway to enhance the CO2 adsorption energetics and uptake in MOFs and covalent organic frameworks (COFs). Markedly, the few reported strategies targeting air capture using MOFs are centred on the aptitude of grafted amines to form a strong chemical bond (at least 70 kJ·mol−1) with CO2, affording high affinity toward CO2 and therefore high CO2 selectivity. Particularly, ethylenediamine (ED) grafting on Mg-MOF-74 supports have been studied for CO2 adsorption from ultra-dilute gas streams such as ambient air. Similarly, N,N-dimethylethylenediamine grafting for DAC using an expanded isostructure of Mg-MOF-74 has also been studied. All such materials suffer from the drawbacks of amine grafted materials as discussed above.